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Two experiments were conducted to evaluate the effect of vitamin A supplementation of a commercial layer diet on the laying performance and immune function of heat-stressed hens. In Experiment 1, two different levels of vitamin A supplementation (3,000 and 9,000 IU/kg) were used to investigate the laying performance and antibody titer against Newcastle disease virus (NDV) of heat-stressed hens. Results showed that the high level of vitamin A supplementation (9,000 IU/kg) had a beneficial effect on the feed intake and laying rate of heat-stressed hens (P < 0.05), compared with the control group (3,000 IU/kg). The antibody titers were not influenced by the level of vitamin A (P > 0.05). In Experiment 2, the effect of four levels of vitamin A (3,000, 6,000, 9,000, and 12,000 IU/kg) on the antibody titer to NDV and T lymphocyte proportion was studied. The experimental birds were exposed to a high temperature (31.5 C) 15 d after NDV vaccination (Treatment 1) or immediately (Treatment 2). The results showed that the egg weight was increased (P < 0.01) by the high levels of vitamin A supplementation (6,000 and 9,000 IU/kg), but feed intake, laying rate, and body weight loss were not (P > 0.05). In Treatment 1, vitamin A had no significant effect on antibody titers against NDV in normal or hot environments but increased (P < 0.01) the proportion of alpha-naphthyl acetate esterase (ANAE)-positive cells. Vitamin A supplementation had a significant effect on NDV antibody titer and ANAE-positive cell proportion in Treatment 2 (P < 0.01). The results of the present study suggested that vitamin A supplementation in commercial layer diets to layer chickens under heat stress was beneficial to laying performance and immune function.
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